Interesting question. You're either paying for gas or kWh, so the real question is which is cheaper and how can you compare the two? I've got a 15 kWh solar system so my kWh costs are very low.

The compare standard MPG vs MPGe the answer is in the MPG equivalent that equates the costs to make it simple to understand. As long as the conversion and assumptions are realistic, you can treat it as 1-to-1, where 25 MPG = 25 MPGe. Then you can add up your savings as shown above. If gas or electricity costs change dramatically the conversions go out the window.

Good explanation here

https://cars.usnews.com/cars-trucks/what-is-mpge explaining different costs so you can get accurate info based on your electricity costs vs buying electricity when away from home.

Low cost of energy in kWh is same as low cost of gas, and vehicle efficiency in miles per kWh is same as efficiency in miles per gallon. To justify a new electric or hybrid ride, you need to find the efficiency of vehicle which is measured in lower values of energy used as kWh /100 km. So, if a bad ass Tesla is 93 MPGe, then I hope a F-150 will come in at (20 mpg x 4) or 80 MPGe meaning fuel cost and efficiency is factor of 4 times cheaper than current gas economy for 3.5 EB at 20 mpg average. Full Electrics rated using MPGe (the equivalent cost to get a full charge and [x] number of miles) make it easy to compare at 1-to-1 for fuel cost per mile. The more you drive the quicker the fuel savings add up.

To keep the theme, in the future I hope to justify why a really cool 2025 Electric F-150 makes sense, with some real numbers showing just how much I'm saving. Same way I rationalized that fuel cost savings in 2.7 vs 3.5 EB was next to nothing compared to benefits of extra HP and torque. Thankfully wife agreed with my logic that 3.5 EB was the way to go. I couldn't quite justify a 3.5 EB vs the new power boost at 24 mpg on fuel costs alone. On fuel Meterman will only save about $70/month going from 18 mpg to 24 mpg. As Mulyhuntr suggested you may need another reason.

-- Cheers, good luck with your decision.